The present disclosure relates to an amplifier that amplifies a high-frequency signal. In the related art, it is known that it is necessary to match the input impedance of an amplifier to a characteristic impedance (of, for example, about 50Ω) to prevent the reflection of an input signal. For example, Japanese Unexamined Patent Application Publication No. 2011-250099 discloses an antenna amplifier device that is provided with a noise figure (NF) matching circuit between an antenna having a capacitive impedance and an amplifier circuit to switch the input impedance to the amplifier circuit in accordance with the receive frequency. The NF matching circuit includes a plurality of coils having different inductances, and a switch for connecting one of the coils that is selected in accordance with the receive frequency between the antenna and the amplifier circuit. Since the NF matching circuit allows the input impedance to the amplifier circuit to be switched in accordance with the receive frequency, a high-frequency signal received by the antenna is amplified with low NF by the amplifier circuit even when an input signal is weak.
In the NF matching circuit disclosed in Japanese Unexamined Patent Application Publication No. 2011-250099, one of the pluralities of coils (inductors) having different inductances is selected in accordance with the frequency of an input signal to increase the bandwidth of the matching circuit. In this case, the inductance necessary for the matching of the input impedance is known to increase as the frequency of the input signal decreases. In the NF matching circuit disclosed in Japanese Unexamined Patent Application Publication No. 2011-250099, the inductors other than the selected inductor are not used for the matching of the input impedance and thus the necessary inductance needs to be implemented with a single inductor. In order to increase the inductance of an inductor, it is typically necessary to increase the number of turns in the inductor. Since there is a limitation in the volume that can be used to form an inductor, it is necessary to reduce the width of a wire that is used to form the inductor to increase the number of turns in order to realize the inductance necessary for the matching of the input impedance while maintaining the element size. The narrower a wire used to form an inductor, the greater the resistance component of the inductor, resulting in increased insertion loss. Thus, in the antenna amplifier device disclosed in Japanese Unexamined Patent Application Publication No. 2011-250099, an increase in the inductance of an inductor selectable in the NF matching circuit to implement an input matching circuit with a wide bandwidth results in an increase in the resistance component of the inductor if there is a constraint on the element size. This can increase the insertion loss of the antenna amplifier device and increase the noise figure, which is a characteristic important for an amplifier. As a result, it may be difficult to implement an input matching circuit with a wide bandwidth and reduced deterioration of the noise figure of the antenna amplifier device.